1. Field of the Invention
This invention relates to a process for removing soap-contaminated conversion layers remaining on metal workpieces after cold-forming by means of aqueous, acid cleaning agents as well as the use thereof for cleaning soap-contaminated calcium aluminate coatings remaining on workpieces of aluminium or aluminium alloys after cold-forming.
In connection with the industrial manufacture of workpieces by means of a non-cutting cold-forming operation, it is for instance necessary to remove remaining lubricant-contaminated phosphate, oxalate or aluminate coatings, so-called soaked-in phosphate, oxalate or aluminate coatings. As lubricant, in particular oils and soaps are used, which chemically react with the previously applied phosphate, oxalate or aluminate coating, thereby forming an intimate bond. The preparation of cold-forming by applying phosphate, oxalate and aluminate coatings and the subsequent treatment with lubricants is commonly used in particular in the field of tube drawing, wire drawing, cold extrusion or sinking.
The above-mentioned soaked-in coatings must be removed above all when it is for instance intended to perform a soft annealing of the workpiece, a further chemical surface treatment or a lacquer coating.
2. Prior Art
The acid cleaning of aluminium, in particular of cold-formed aluminium is known and described for instance in U.S. Pat. No. 3,969,135 with respect to the cleaning of aluminium cans provided with lubricant residues, which cans were produced by deep-drawing or sinking from thin round sheet metal blanks. As a basis of the acid cleaning agent, there is used sulfuric acid with a minor addition of hydrofluoric acid. A further acid cleaning agent for aluminium or aluminium alloys is described in EP-A-275,043. As basis there is used a mineral acid from the group including sulfuric acid, phosphoric acid and nitric acid. Both the aforementioned processes have in common that the aluminium or aluminium alloy surface was formed without preceding conversion treatment by merely using lubricant.
The DE-A-3,843,148, however, describes a process for removing soaked-in conversion and soap layers with alkaline cleaning agents and under the influence of ultrasound. After a correspondingly high load, the constituents of the detached conversion layers accumulate in the cleaning bath, and the bath must be disposed of, which is generally effected by means of a breakdown with acid. It is a disadvantage of this process that major amounts of a strongly alkaline solution must not only be neutralized, but also greatly acidified. In the alkaline cleaning especially of aluminium or aluminium alloys it is furthermore disadvantageous that when using strongly alkaline cleaning agents the material is solubilized, or when using mild alkaline cleaning agents, an only very slow and incomplete detachment of the lubricant-contaminated conversion layer is effected even under the influence of ultrasound.